Does Scala/sbt still take 5 years to compile moderately sized apps?
The designers basically removed everything that made Scala slow to process and hard to optimize. As a side-effect, Kotlin is much easier to read and understand than Scala.
(E.g. as applications get larger you tend to need to understand the reasons for why something didn't happen, so you move from Option on to using an Either-style type that carries some information in the failure case. But Kotlin is hardcoded around using null to express absence and it's impossible to migrate from that to something that includes a reason).
No. It's not.
Also, I notice that, as I write type-safe code, I don't usually compile that often and, when it compiles, it is likely correct.
The incremental compilation is pretty good.